A disc brake has a disc and brake pads, and generally each brake pad includes a lining (friction material) for braking the disc and a back plate for supporting the lining. Since this back plate supports the lining, it is required to have heat resistance, brake resistance, and high mechanical strength in a high temperature atmosphere. For this reason, conventionally, plates made of ceramic or plates made of metal have been used for the back plate. However, when the plates made of ceramic and the plates made of metal are used for the back plate, there are problems such as a heavy weight, a long time required for machining, high costs, and the like.
Therefore, recently, it is attempted to use a plate made of a synthetic resin mixed with fibers for the back plate instead of the plates made of metal for the purpose of reducing both weight and cost.
As technology relating to this type of back plate, Patent Document 1 discloses a back plate that uses a carbon fiber reinforced plastic plate.
Here, the disc of the disc brake is generally formed into a circular shape (a disc shape). By making the lining of the brake pad contact with the disc (a surface of the disc), a rotation of the disc is braked due to a frictional force generated between the brake pad and the disc. When the disc rotates, a relative velocity of the disc with regard to the brake pad on the side of an outer circumference of the disc is higher than the relative velocity thereof on the side of a center of the disc. Therefore, when the rotation of the disc is braked, a higher frictional force (a braking force) has to be applied to a region of the brake pad (the lining) on the side of the outer circumference of the disc as compared with a region of the brake pad on the side of the center of the disc.
However, in a conventional brake pad, each of the lining and the back plate of the conventional brake pad has a substantially uniform thickness. In the case where such a brake pad is continuously used, a phenomenon (an uneven wear) that the region of the lining on the side of the outer circumference of the disc is worn earlier than the region of the lining on the side of the center of the disc occurs. If a degree of the wear of the lining is larger, there is a case that a part of the back plate is exposed from the lining. The exposure of the back plate causes a problem in that the frictional force of the brake pad is reduced.